The present invention relates to a method for installing an underwater riser, or vertical tower, to transport hydrocarbons between the seabed and a surface of the water.
Underwater risers are known and they are installed between a bottom installation situated close to a well and a surface installation situated substantially vertically above the bottom installation. They mainly comprise, on the one hand, a vertical tubular pipe, flexible or else rigid, which extends from the bottom installation to a subsurface level where it is retained by means of a float, and, on the other hand, a flexible pipe connected to the tubular pipe at the level of the float and which extends from the float to join the surface installation. The normative documents API-RP-17B show underwater risers consisting only of flexible pipes. So-called “hybrid tower” risers, in which the vertical tubular pipe is a rigid pipe, are also implemented.
The float is generally installed well below the surface, in an area where the movements of the water are smaller compared to those of the surface. The tubular pipe has a bottom end connected to the bottom installation and a top end held through the float and connected to the flexible pipe. Thus, the hydrocarbon can be raised from the bottom installation to the surface installation through the vertical pipe and the flexible pipe.
Given the weight of the elements present, and the marine environment, the installation of these underwater risers is costly in terms of time and energy. Furthermore, the precise adjustment of the bottom end of the vertical tubular pipe on the bottom installation is difficult precisely because of the surface movements.
Also, in the interests of freedom from said surface movements, consideration has been given, on the one hand, to flooding the float to submerge it with the tubular pipe and, on the other hand, to installing a pulling cable at said bottom end of the pipe, to engaging this cable in return means situated on the bottom installation and to pulling it by means of another float submerged below the surface, so as to drive the bottom end toward the bottom installation. For this, the flooded float is gradually emptied to take up the pulling forces of the cable and to accurately adjust the bottom end on the bottom installation. The adjustment of the relative position of the two floats which is performed below the moving surface is easier and, consequently, the connection of the bottom end to the bottom installation is performed in better conditions.
Reference can be made to the document FR 2 927 651, which describes such an installation method.
However, in some installation conditions, and in particular when great water depths are involved, the tubular pipe is relatively long and the float is consequently more bulky. Also, their handling requires crane handling means that are adapted accordingly and relatively heavy for the surface installations.
Also, one problem which arises and that the present invention aims to resolve, is how to provide an underwater riser installation method that does not require particular additional installation means and that nevertheless makes it possible to install very long tubular pipes and their associated float.